This is the National Phase Application of PCT/EP99/00236, filed Jan. 16, 1999.
The present invention relates to a method for producing particularly advantageous catalysts which contain ruthenium and at least one other metal with an atomic number in the range from 23 to 82, and to the use, of such catalysts for hydrogenations.
It is known that carboxylic acids can be reduced catalytically with hydrogen to alcohols. A summarizing description thereof is given in Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], 4th edition, vol. VI/1b, pages 103-107 (1984). Where catalysts containing ruthenium are employed (see page 106), relatively high temperatures and very high pressures are necessary, for example temperatures of 145 to 190xc2x0 C. and pressures of 700 to 950 bar.
J. Org. Chem. 24; 1847 (1959) describes a method for the hydrogenation of carboxylic acids in which rhenium oxide is employed as catalyst. However, long reaction times and high pressures are required. In addition, the yield and the selectivity are unsatisfactory.
Catalysts which, besides ruthenium, contain other metals, for example rhenium, are also known for various hydrogenations. Such multimetallic catalysts may be in Raney form, in sponge form or as supported catalysts. Thus, EP-A 724 908 describes Raney ruthenium catalysts which are modified by other metals and can be employed for various hydrogenations. However, they require large catalyst weights and long reaction times. The Raney ruthenium is prepared by leaching the aluminium out of a ruthenium/aluminium alloy with sodium hydroxide solution. This results in the Raney ruthenium catalyst containing residues of aluminium, and aluminium oxides and sodium hydroxide solution. This has adverse effects on the catalyst performance.
In addition, the handling of such catalysts is particularly complicated because they are pyrophoric.
DE-A 195 10 438 describes a method for producing 1,4-butanediol and tetrahydro-furan from furan, it being possible to employ as catalysts inter alia ruthenium/rhenium sponge. This can be obtained by reducing an aqueous Re2O7/RuCl3 solution. Such catalysts have only unsatisfactory activities and selectivities.
WO 96/27436 describes catalysts containing ruthenium and rhenium which are applied to a support material consisting of carbon. The disadvantage of these catalysts is that for hydrogenations they require temperatures and pressures at which racemizations occur to a considerable extent on use of optically active precursors.
The preparation of ruthenium oxide of high purity with a high specific surface area is described in DE-A 2 132 547. This monometallic catalyst shows only low activities in the hydrogenation of carboxylic acids, however. In addition, if optically active precursors are employed, racemizations occur to a considerable extent.
A method for producing catalysts which contain ruthenium and at least one other metal with an atomic number in the range from 23 to 82 has now been found and is characterized in that a suspension of a ruthenium compound which has a specific surface area in the range from 50 to 300 m2/g is mixed with a solution of at least one metal compound where at least one of these metal compounds contains a metal different from ruthenium and having an atomic number in the range from 23 to 82, and the mixture of suspension and solution is treated with a reducing agent.
According to the present invention it is possible to produce, for example, bimetallic ruthenium/metal X catalysts and trimetallic ruthenium/metal X/metal Y catalysts. Catalysts produced according to the invention contain ruthenium and, for example, the metals X and, where appropriate, Y in elemental form and may be, for example, in the form of a colloid or in the form of intermetallic compounds. Examples of metals X and Y are rhenium, osmium, iron, cobalt, rhodium, palladium, platinum, copper, zinc, silver, tin, germanium, gallium, lead and tin. It is preferred according to the invention to produce catalysts which, besides ruthenium, contain one of the said metals or, besides ruthenium, contain combinations of rhenium with copper, silver or tin. Ruthenium/rhenium catalysts are particularly preferably produced.
The catalysts may contain the individual metal components in any ratios. Preferred catalysts are those in which the individual metals are present in equal atomic ratios, or ruthenium is present in excess. For ruthenium/metal X catalysts, for example, an Ru/X atomic ratio of 1/1 to 10/1, in particular of 1/1 to 5/1, is preferred.
A wide variety of suspending agents are suitable for preparing the suspension of a ruthenium compound. Water is preferred in this connection. Particularly suitable ruthenium compounds are those which can be reduced to the metal in a simple manner, for example with hydrogen. The ruthenium compounds may have various oxidation states, for example +3, +4 or +8. Ruthenium oxides and ruthenium oxide hydrates in which the ruthenium is present in the +4 oxidation state are preferred. Aqueous suspensions containing ruthenium oxide or ruthenium oxide hydrate of the necessary specific surface area are commercially available or can be produced in a known manner, for example by dissolving a water-soluble ruthenium compound, for example ruthenium trichloride, in water, making the solution alkaline and adding an oxidizing agent. A ruthenium (IV) oxide hydrate with a suitable specific surface area then precipitates. It is then possible by decantation, addition of water and acid and filtration to obtain an aqueous suspension of ruthenium (IV) oxide or oxide hydrate which can be employed directly for the catalyst production according to the invention.
Suitable suspensions of ruthenium compounds which have a specific surface area in the range from 50 to 300 m2/g and arc suitable for the production according to the invention of catalysts arc suspensions which contain, for example, 0.01 to 200% by weight of ruthenium, calculated as metal. These suspensions preferably contain 0.1 to 4% by weight of ruthenium. Preferred suspensions contain ruthenium compounds with a specific surface area in the range from 100 to 250 m2/g.
The solution which contains at least one metal compound in which at least one metal different from ruthenium and having an atomic number in the range from 23 to 82 is present may contain as solvent for example those which are miscible with the suspending agent for the ruthenium compound. If the suspending agent for the ruthenium compound is, for example, water, the solvent for the other metal compound(s) may be, for example, water or C1-C4-alkyl alcohols. The suspending agent and the solvent is preferably water. Metal compounds which contain at least one metal different from ruthenium and having an atomic number in the range from 23 to 82 may be, for example, compounds, which are soluble in the particular solvent, of the metals which are indicated above as examples of metals X and Y. Aqueous solutions of rhenium compounds are preferably used, in particular aqueous solutions of rhenium heptoxide (Re2O7), which are also referred to as perrhenic acid (HReO4 or Re2O7xc3x972 H2O).
The solution which contains at least one metal compound in which at least one metal different from ruthenium and having an atomic number in the range from 23 to 82 is present may contain, for example, 0.01 to 60% by weight of the metal compound(s), calculated as metal. This content is preferably 0.1 to 5% by weight.
It is also possible to proceed in such a way that the solution which contains at least one metal compound in which at least one metal different from ruthenium and having an atomic number in the range from 23 to 82 is present is generated in situ. For example, the metal compound(s) can be added in solid form to the suspension of a ruthenium compound with the necessary specific surface area and, where appropriate, additional solvent. Formation of the solution of the metal compound(s) and their mixing with the suspension of the ruthenium compound then take place simultaneously.
Treatment of the mixture of suspension and solution with a reducing agent can in principle be carried out with all reducing agents which are able to reduce the ruthenium compound and the other metal compound(s). It has proved advantageous in many cases, especially because virtually no interfering by-products are formed, to treat with hydrogen at temperatures in:the range from 50 to 200xc2x0 C. and pressures in the range from 5 to 200 bar.
Since the catalysts produced according to the invention can preferably be employed for hydrogenations, the previously described reduction with hydrogen may also be carried out in the reaction vessel for the hydrogenation, preferably immediately before the hydrogenation itself.
It is also possible to proceed in such a way that a suspension of a ruthenium compound is reduced before the mixing with the solution of one or more compounds of other metals, and the suspension and the solution are reduced once again after the mixing.
Catalysts produced according to the invention show intimate contact of the metals present therein and specific surface areas of, for example, 50 to 150 m2/g. The individual catalyst particles generally contain all the metals whose compounds were employed in the production of the catalyst. Virtually no particles which contain only one metal, for example only ruthenium, are present. This has emerged from microanalytical and electron microscopic investigations.
Catalysts produced according to the invention have high activity. This is surprisingly significantly higher than for catalysts of identical composition produced by reducing a solution containing a ruthenium compound and at least one metal compound of a metal different from ruthenium and having an atomic number in the range from 23 to 82 (see Example 5).
Catalysts produced according to the invention can be used in particular for hydrogenations. Important possible uses are, for example, the production of alcohols which are optically active where appropriate from carboxylic acids, carboxylic esters and carboxamides which are optically active where appropriate; the preparation of amino alcohols which are optically active where appropriate from amino acids, esters and amides which are optically active where appropriate, the preparation of alcohols from carboxylic acids, in particular of saturated diols from saturated or unsaturated dicarboxylic acids, dicarboxylic esters and dicarboxamides and the preparation of alcohols which are optically active where appropriate from hydroxy carboxylic acids, hydroxy carboxylic esters and hydroxy carboxamides which are optically active where appropriate. Individual examples are the preparation of L-1,2,4-butanetriol from L-malic acid, of L-alaninol from L-alanine, of n-butanol and 1,4-butanediol from maleic acid and of 1,6-hexanediol from adipic acid. Hydrogenations with catalysts produced according to the invention can be carried out, for example, at 0 to 120xc2x0 C. and 5 to 300 bar.
Catalysts produced according to the invention can be modified, where appropriate by treatment with sulphur compounds, for example thioethers.
Catalysts produced according to the invention are free of support materials, aluminium, aluminium oxides and alkalis.
Catalysts produced according to the invention are distinguished by the possibility of carrying out with them hydrogenations under milder conditions, in higher yields, with substantial retention of optical activities and/or with negligible impairment of functional groups otherwise present. In addition, not particularly large amounts of catalyst arc required, and the reaction times are generally distinctly shorter than with ruthenium-containing catalysts of the state of the art.
The present invention also relates to ruthenium-containing catalysts which are free of support materials and which are obtainable by mixing a suspension of a ruthenium compound which has a specific surface area in the range from 50 to 300 m2/g with a solution of at least one metal compound where at least one of these metal compounds contains a metal different from ruthenium and having an atomic number in the range from 23 to 82, and the mixture of suspension and solution being treated with a reducing agent.